The present invention relates to pressure measurement in conjunction with mass spectrometry, and in particular to continuous pressure measurement by monitoring of ion beam current.
Mass spectrometry is now widely used in a variety of fields. In particular, it is used to monitor and control a number of processes in semiconductor fabrication, including chemical and physical vapor deposition and ion implantation. Accurate measurement and control of fabrication processes is essential in semiconductor manufacture. One important parameter to be measured in many processes is the total chamber pressure.
Pressure in vacuum chambers can be measured using a variety of devices, including thermocouple gauges, ionization gauges, rotating-disc viscosity gauges, and others. The type of gauge used in any particular system is generally selected according to considerations of cost, desired accuracy, and expected pressure range. Typically, pressures in the millitorr range are measured with a thermocouple gauge or other gauge that measures heat dissipation, whose readout times depend on the mass of the heated element and the heat transfer coefficient. These gauges cannot practically deliver pressure data at rates faster than a few Hz. Even commercial ultra low pressure ion beam gauges provide discrete readouts at only moderate frequencies, e.g., about 1 Hz.
Semiconductor manufacturing lines currently do not dynamically use pressure data to control deposition rates or other process parameters. While pressure may be monitored during the fabrication process, the low data rates of the pressure gauges described above make it infeasible to use them for process control.
Much faster pressure readings can be obtained by monitoring the primary ion beam current in the collector of a mass spectrometer. The ion beam current is related to the ion source pressure, which can in turn be related to chamber pressure. Such a system is described, for example, in U.S. Pat. No. 5,834,770 to Holkeboer et al. Unfortunately, pressure readings obtained in this way are often not particularly stable. xe2x80x9cDriftxe2x80x9d in the relationship between beam current and ion source pressure can occur as the source parameters are varied (e.g., to tune the instrument), as the source becomes dirty, as the source filament ages, or for other reasons. Thus, primary beam current is rarely used to monitor pressure in these systems.
A need therefore still exists for a system which can provide fast (preferably continuous), accurate pressure measurement in mass spectrometry systems.
The present invention allows rapid monitoring of pressure in a chamber equipped with a mass spectrometer by using the primary beam current in conjunction with a conventional pressure gauge. The conventional gauge allows frequent calibration of the relationship of the beam current to the chamber pressure, preventing excessive drift in the system. An advantage of the system is that it takes advantage of instruments already present in a typical spectrometry apparatus.
In one aspect, the invention comprises apparatus for monitoring chamber pressure, including an ion source which produces an ion beam, a collector where beam current can be continuously measured, and a pressure gauge which can be used to obtain a series of discrete pressure measurements (e.g., at a rate of less than about 100 Hz, or at a rate of less than about 1 Hz). The apparatus further comprises analysis means which combine the beam current measurement and the pressure gauge measurements to continuously determine the chamber pressure, for example by using the beam current to determine an offset to the most recently measured gauge pressure. The apparatus may comprise orthogonal acceleration means that allow the ion beam to be used in time-of-flight mass spectrometry, or the ion beam may be a component of another type of mass spectrometer.
In another aspect, the invention comprises methods of monitoring chamber pressure in a chamber equipped with a pressure gauge, an ion source that produces an ion beam, and an ion collector from which a beam current can be measured. The methods include monitoring the pressure gauge to obtain a series of discrete pressure measurements (e.g., at a rate of less than about 100 Hz, or at a rate of less than about 1 Hz), and continuously monitoring the beam current (e.g., at a rate of at least about 1 kHz, 10 kHz, or 50 kHz). The continuous beam current measurement and the discrete pressure measurements are then combined to yield a continuous chamber pressure measurement. The continuous pressure measurement may serve as an input to an automated process control system.